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// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build !386
// TODO finish intrinsifying 386, deadcode the assembly, remove build tags, merge w/ intrinsics_common
// TODO replace all uses of CtzXX with TrailingZerosXX; they are the same.
package sys
// Using techniques from http://supertech.csail.mit.edu/papers/debruijn.pdf
const deBruijn64ctz = 0x0218a392cd3d5dbf
var deBruijnIdx64ctz = [64]byte{
0, 1, 2, 7, 3, 13, 8, 19,
4, 25, 14, 28, 9, 34, 20, 40,
5, 17, 26, 38, 15, 46, 29, 48,
10, 31, 35, 54, 21, 50, 41, 57,
63, 6, 12, 18, 24, 27, 33, 39,
16, 37, 45, 47, 30, 53, 49, 56,
62, 11, 23, 32, 36, 44, 52, 55,
61, 22, 43, 51, 60, 42, 59, 58,
}
const deBruijn32ctz = 0x04653adf
var deBruijnIdx32ctz = [32]byte{
0, 1, 2, 6, 3, 11, 7, 16,
4, 14, 12, 21, 8, 23, 17, 26,
31, 5, 10, 15, 13, 20, 22, 25,
30, 9, 19, 24, 29, 18, 28, 27,
}
// Ctz64 counts trailing (low-order) zeroes,
// and if all are zero, then 64.
func Ctz64(x uint64) int {
x &= -x // isolate low-order bit
y := x * deBruijn64ctz >> 58 // extract part of deBruijn sequence
i := int(deBruijnIdx64ctz[y]) // convert to bit index
z := int((x - 1) >> 57 & 64) // adjustment if zero
return i + z
}
// Ctz32 counts trailing (low-order) zeroes,
// and if all are zero, then 32.
func Ctz32(x uint32) int {
x &= -x // isolate low-order bit
y := x * deBruijn32ctz >> 27 // extract part of deBruijn sequence
i := int(deBruijnIdx32ctz[y]) // convert to bit index
z := int((x - 1) >> 26 & 32) // adjustment if zero
return i + z
}
// Ctz8 returns the number of trailing zero bits in x; the result is 8 for x == 0.
func Ctz8(x uint8) int {
return int(ntz8tab[x])
}
// Bswap64 returns its input with byte order reversed
// 0x0102030405060708 -> 0x0807060504030201
func Bswap64(x uint64) uint64 {
c8 := uint64(0x00ff00ff00ff00ff)
a := x >> 8 & c8
b := (x & c8) << 8
x = a | b
c16 := uint64(0x0000ffff0000ffff)
a = x >> 16 & c16
b = (x & c16) << 16
x = a | b
c32 := uint64(0x00000000ffffffff)
a = x >> 32 & c32
b = (x & c32) << 32
x = a | b
return x
}
// Bswap32 returns its input with byte order reversed
// 0x01020304 -> 0x04030201
func Bswap32(x uint32) uint32 {
c8 := uint32(0x00ff00ff)
a := x >> 8 & c8
b := (x & c8) << 8
x = a | b
c16 := uint32(0x0000ffff)
a = x >> 16 & c16
b = (x & c16) << 16
x = a | b
return x
}
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